Almost 2.5 million people visit the emergency room each year in the United States as a consequence of sustaining a traumatic brain injury (TBI). Of these almost 75% are diagnosed with a mild TBI or a concussion, and almost a third of these patients go on to develop long-term behavioral problems. Executive function deficits, emotional disturbances such as depression and anxiety, affective disorders and substance use disorders are some of more common complaints of mild TBI patients. Adolescent boys and girls (high school and college-age) are more severely affected by concussions compared to older (adult) patients. Emerging data also suggest that girls and women are twice as likely to sustain a mild TBI, have different and more severe symptoms and take longer to recover from than their male counterparts. We have developed a model of mild TBI in the adolescent (5-week-old) rat and have demonstrated that despite similar extents of axonal injury in the early post-traumatic period only the female rats exhibit cognitive deficits. Importantly, as these animals age into adulthood, they begin to develop depression-like behavior, which manifests only in the estrus phase of the estrous cycle. This phenomenon of transient helplessness and anhedonia are hallmarks of premenstrual dysphoric disorder that some women experience. Our preliminary data demonstrate that activating the mesocorticolimbic dopamine circuit using chemogenetic approaches or agonist of the D2 receptor reversed the depressive-like behavior observed in injured animals. The activity of the D2 receptor varies with phases of the estrous cycle with the lowest activity occurring immediately after the hormone surge that occurs during proestrus. Blocking this surge using estrogen and progesterone receptor antagonists was able to reduce depressive-like behavior. The working hypothesis of this proposal is that mild TBI in the adolescent female rat results in the development of a hypodopaminergic state characterized by a decrease in the activity of the dopamine neurons in the ventral tegmental area (VTA) and the expression of D2 receptors in the medial prefrontal cortex (PFC). The specific aims are designed to test whether the estrogen receptor ? isotype is the key mediator of the decrease in D2 receptor expression (Aim 1), whether the activation of D2 receptors within the medial PFC facilitate the reversal of the depressive-like behavior (Aim 2), and whether the activation of the DA neurons in the VTA projecting to the medial PFC will reverse depressive-like behavior. Hormone receptor antagonists will be systemically administered, D2 receptor agonist and retroAAV2 will be infused directly into the medial PFC, and Gq-designer receptors exclusively activated by designer drugs will be expressed in the VTA using an AAV vector. The data from these experiments will provide the mechanistic basis for sex-specific behavioral deficits following mild TBI sustained in adolescence.